1. Field of the Invention
This invention relates to new varnish polyisocyanates having isocyanurate or urethane groups, which are blocked by blocking agents, and which have as a base 2-methyl-1,5-pentanediisocyanate (MPDI), 2-ethyl-1,4-butanediisocyanate (EBDI) or a mixture thereof. This invention also relates to the use of these polyisocyanates as an isocyanate component for the production of polyurethane powder varnishes that are thermosetting.
2. Description of the Prior Art
Polyurethane powder varnishes with a base of partially or totally blocked polyisocyanates and polymers exhibiting hydroxyl groups, whose softening point is above 40.degree. C., belong to the state of the art and are described many times in the literature, e.g., in DE-OSS Nos. 21 05 777, 25 42 191, 27 35 497, 28 42 614, 30 04 876, 30 39 824 or 31 28 743.
Polyurethane powder varnishes consist essentially of a component, which carries hydroxyl groups, and a polyisocyanate, whose NCO groups are blocked partially or completely with a blocking agent, so that the OH/NCO polyaddition reaction cannot proceed at temperatures under 140.degree. C. Only after heating to temperatures .gtoreq.150.degree. C. can powder varnishes of this type cross-link within a relevant time with release of the blocking agent and reaction of the OH groups with the NCO groups to form a varnish film.
Not all polyisocyanates are suitable for this purpose and with a view to use as powder materials there are quite a few limitations, especially, when the polyisocyanates, blocked partially or totally with the usual blocking agents, themselves exhibit such characteristics that make grindability difficult. Thus, their use in powder varnishes is limited to a great extent, if indeed not impossible, no matter how good their varnish technical, chemical and physical characteristics may be.
Thus, many attempts have been made to use hexamethylene-1,6-diisocyanate (HDI) and/or its adducts in blocked form for polyurethane varnishes. In DE-OS No. 30 33 860 or EP No. 0 047 452, page 8, it is correctly observed that products with a preponderant portion of HDI (in this case, it is mixed trimers of HDI and isophoronediisocyanate), therefore, in other words, pure HDI trimers, are liquid products at room temperature. It is correctly observed on page 19 that it is impossible to use modified HDI as powder hardening agents exclusively by formation of isocyanurate groups, regardless of the degree of trimerization. A combination of HDI/IPDI was found to be necessary for raising the melting point for use as powder varnishes.
Hence, a need clearly continues to exist for polyisocyanates which, although partially or totally blocked, can be used effectively in polyurethane varnishes. In particular, a need continues to exist for polyisocyanates which, although partially or totally blocked, are produced in a brittle, grindable form which can be used in powder varnishes.